Prosthetic and orthotic devices help restore mobility to people who lack able-bodied motion or gait. Prosthetic devices are intended to replace the function or appearance of a missing limb and can return mobility to the wearer or user. Orthotic devices are intended to support or supplement an existing limb, by assisting with movement, reducing weight-bearing loads on the body, reducing pain, and controlling or restricting movement. Prosthetic and orthotic devices are available to replace or support various portions of the body. Lower limb prosthetic devices include, for example, the prosthetic foot, the foot-ankle prosthesis, the prosthetic knee joint, and the prosthetic hip joint. Lower limb orthotic devices include, for example, the foot orthoses, the ankle-foot orthoses, the knee-ankle-foot orthoses, and the knee orthoses. People who require a lower limb prosthesis or orthosis often expend more metabolic power to walk or move at the same speed as able-bodied individuals. One goal of lower limb prosthetic and orthotic devices is to help the user achieve a normal gait while reducing energy expended by the user.
Prosthetic and orthotic devices can be divided into two groups, passive devices and active devices. Passive lower limb prosthetics generally rely on compliant members, such as springs, to store and release energy. A spring is able to return only as much energy as is put into the spring. Thus, the energy that is released by a spring in a passive device is limited to the energy that is put in by the user. For example, a spring-based passive foot prosthetic provides about half of the peak power required for gait. The user of a passive device must expend additional energy through other muscles and joints to maintain a normal walking gait. Therefore, passive prosthetic and orthotic designs are limited in capacity to help users reduce metabolic energy expenditure while achieving a normal walking gait and performing other activities.
Active devices differ from passive devices in that active devices use a motor to supply power to the device and to control the device. Current active device designs are inefficient, either requiring relatively large motors, which are heavy and undesirable for wearable devices, or providing low peak power output, which is insufficient for many activities. Control systems for active devices are limited in capability to control active devices. Active prosthetics are typically restricted to a single degree of freedom, which reduces the motion available to the device. Further, active prosthetics are limited to low power activities, because the power necessary for high power activities is unattainable in a small portable system. One goal of active device designs is to increase efficiency of the active components and to build a lighter weight device.
Prosthetic devices are typically designed for a specific activity, such as walking. The majority of active compliant devices utilize a traditional rigid structure. The traditional rigid structure typically includes links powered by actuators such as electric motors or hydraulics. One strategy employs an architecture having a joint which is powered by a compliant member, such as a spring, and an active member, such as a motor driven screw, arranged in series. An activity-specific design strategy and traditional rigid structures may be suited for one specific activity, but the designs are limited in application and are not efficient beyond the intended activity. For example, devices designed for walking perform poorly for running, navigating uneven terrain, walking up and down inclines or stairs, or simply balancing while standing. Carrying heavy loads or transitioning from walking to running remains a challenge for users. Current active devices are ineffective for activities requiring both high velocities under low load and low velocities under high load. Another goal of prosthetic device designs is to perform more similarly to a human muscle during a variety of activities.